well reproduces the experiment in the whole temperature range (Fig. REF b).
Here, the first term is due to spin fluctuations in the CEF ground-state Kramers' doublet, while the second term is due to the Orbach process inducing temperature-dependent spin fluctuations with frequency given by equation (REF ) [1]}, as found also with ESR.
The fit gives \(\lambda _0=2.1(1)\,\mu \) s\(^{-1}\) , \(\eta =52(8)\)  \(\mu \) s, and \(\Delta _{\mu \text{SR}}=5.7(5)\)  meV.
We note that single-phonon and Raman-scattering contributions, proportional to \(T\)  and \(T^9\)  [2]}, respectively, are too steep in the relevant temperature ranges, and are thus not relevant in NdTa\(_7\) O\(_{19}\) .
